This invention relates to a strut type automatic gap regulator for a drum brake.
A known automatic gap regulator as disclosed in U.S. Pat. No. 3,874,481 is shown in FIG. 3. More specifically, a platelike strut 103 is provided between a pair of conventional brake shoes 101 and 102 arranged opposite to each other and fitted to a backing plate 100. A cam member 105 is coupled to one end of the strut 103 through a leaf spring 104, the cam member 105 being provided with a toothed portion 105a for engaging a gear member 106 fixed to the strut 103. A projection 105b extends into a through-hole 101a formed in the web of one brake shoe 101 with a gap .delta..sub.o therebetween. The other end of the strut 103 is mated with the web of the other brake shoe 102 through a rocking parking lever 108 coupled thereto by a pin 107. A coil extension spring 109 biases the strut 103 toward the other brake shoe 102.
During the normal operation of the brake, hydraulic oil is introduced from an external hydraulic oil source into a wheel cylinder (not shown) to expand both the brake shoes 101 and 102. The strut 103, together with the brake shoe 102, is then moved to the left on the drawing by the action of the coil extension spring 109, if the shoe gap exceeds the normal shoe gap because of the wear of the linings, to fill up the gap .delta..sub.o between the projection 105b of the cam member 105 and the through-hole 101a. Then the cam member 105 is dragged by the brake shoe 101 to the right relative to the diagram and the toothed portion 105a disengages from the gear member 106. In consequence, the cam member 105 is slightly revolved clockwise around the fulcrum O.sub.o, and the projection 105b is thereby tilted to the right and caused to engage with the gear member 106 with one tooth in the toothed portion advanced. The shoe gap is thus automatically regulated.
The gap .delta..sub.o between the projection 105b fitted to the cam member 105 and the through-hole formed in the web of the brake shoe is the ordinary shoe gap and, since the strut 103 is practically extended by revolving the cam member 105, the gap .delta..sub.o tends to be unstable as cam member 105 revolves. Moreover, the transmission of thrust to the brake shoes through the strut 103 is not linear. Therefore, the size of the strut 103 and the cam member 105 must be increased.
These problems are overcome according to the present invention in which toothed portions of a strut engage similar toothed portions of a latch which engages a shoe for linear movement therewith. A first spring extends between the one shoe and the latch to urge the toothed portions together while a second spring extends between the strut and the one shoe to bias the strut in a direction to extend its effective length.
More particularly, a pair of brake shoes are mounted opposite to each other and fitted to a backing plate with a wheel cylinder held between one pair of ends of the brake shoes, the other pair of ends being supported with an anchor. The strut has a fulcrum at one end and a toothed portion at the other, the fulcrum abutting against the web of one brake shoe to hold the web. The strut is biased for rotation by a first spring extending from the strut to the one brake shoe to make the strut extend its effective length. The web of the other brake shoe fits into a guide groove formed in a latch also having a projection inserted into a through-hole bored in the web to provide the gap between the projection and the through-hole. A second spring extends from the latch to the one brake shoe to engage the toothed portion of the latch and that of the strut in a normal state.
If introduction of hydraulic oil from an external hydraulic oil source into the wheel cylinder to actuate the brake for normal use causes the shoe gap to become greater than the shoe gap for normal use because of the wear of the linings of the brake shoes, the brake shoes expand the space between the respective ends thereof. The other brake shoe works to fill up the gap between the through-hole and the projection of the latch and moves with the latch, whereas the one brake shoe moves with the strut. When the toothed portion of the latch is disengaged from that of the strut, the strut is revolved by the coil extension spring stretched to the one brake shoe in such a direction as to extend its effective length. When the brake is then released both the brake shoes revert to the original positions.
The latch is thereby drawn by the coil extension spring stretched over the one brake shoe and changes its position of engagement with the strut and engages therewith again after extending the effective length of the strut to secure the predetermined gap .delta..sub.o between the projection of the latch and the through-hole of the other brake shoe. The shoe gap is thus automatically regulated.